Diamond abrasive article



Jan. 24, 1950 HUNTER ET AL 2,495,257

DIAMOND ABRASIVE ARTICLE Filed June 18, 1947 3mm I RUEERT H. HUNTER WALLACE H. FR/IBEE'G Patented Jan. 24, 1950 DIAMOND ABRASIVE ARTICLE Robert H. Hunter and Wallace 11. Friberg, Au-

burn, Mass., assignors to Norton Company, Worcester, Mass., a corporation of Massachusctts Application June 18, 1947, Serial No. 755,502

Claims.

The invention relates to diamond grindin wheels and other abrasive products, such as honing and lapping sticks and tools.

One object of the invention is to provide a hard bond for diamonds which nevertheless gives a free cutting wheel. Another object is to provide diamond grinding wheels of controlled porosity. Another object is to provide a diamond wheel which has some of the better characteristics of metal bonded diamond'wheels and some of the better characteristics of vitrified bonded diamond wheels.

Another object of the invention is to provide a metal and method of making a diamond wheel therewith that eliminates the use of special atmospheres, ordinary air being used. Another object is to provide a metal and a method of making a diamond wheel therewith in an atmosphere of steam, which is readily available.

Other objects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the features of construction, combinations of elements, arrangements of parts and in the several steps and relation and order of each of said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawing the single figure is a photoinicrograph of a typical bond according to the invention.

The abrasive is diamond, and for the manufacture of a grinding wheel diamond powder will be used. Bort is the name of true crystalline diamond which, however, is usually not of gem quality. Such bort may be crushed to a powder and then screened to separate the particles of different sizes. We can use such material and al m ll cu n s from gems. The invention is not limited to any particular grit size of the diamond powder.

The bond is iron oxide not excluding some alpha iron. The oxide may be FeO or FezOs or Fe:04 or any combination of these. However it is preferred that there shall be a substantial quantity of the magnetic iron oxide, F8304. The presence of other metals and compounds or fillers which are not metals or compounds thereof is not excluded and in some cases will be beneficial. The major portion of the bond by volume however is iron oxide. In many cases the bond around individual diamonds will vary in composition from outer layers to inner layers.

Any size Or shape of grinding wheel or other abrasive article can be made in accordance with the present invention. So-called straight" wheels which are simply discs can be made. These will usually consist of an outer rim of bonded diamond material integral with a central portion of some other grit preferably bonded with the same bond. Cup shapedwheels may be made by forming in one piece an annulus the major portion of which is non-diamond grit bonded by the bond and which is integral with a side face of diamond abrasive bonded with the bond of the invention. Such an annulus may be cemented to a suitable back for mounting on a spindle. Mounted points may be made in accordance with the invention. In commercial practice the diamond containing portion of grinding wheels is limited to a small depth on the rim of straight wheels and on the side face of the annulus of the cup shaped wheels. This is on account of the high cost of diamonds.

We will now give examples for the manufacture of diamond abrasives according to the present invention. In the following tables the term "abrasive section refers to the diamond containing portion of the wheel or other abrasive article while the term base section" refers to the non-diamond containing portion which is molded integral with the abrasive section. In these tables the heading Material refers to the ingredients of the mixture from which the section was molded, and the term pores" refers to the porosity of the article after it was molded but before it was fired. In the tables the heading Volume Per Cent means the percentage by volume as indicated of the various materials and of the pores after molding but before firing. In the tables the heading Weight-grams means the indicated weight of the various materials added in making up the mixture.

EXAMPLE 1 Abrasive section Volume Weight- Mater {31 Per Cent grams Diamond Grit 93.1 11.14 Iron Powder. 53. 8 58.10 Pores 23. l

Base section Volume Weight- Material Per Cent grams Two wheels were-made according to the abo e example. In both cases an annulus for a cup wheel was made which was 6.930" in outside diameter and 4.43" inside diameter and having a total thickness of .375" of which the abrasive section was thick. Both annuluses were molded in a closed mold. Both annuluses were fired in a steam atmosphere. The first wheel was fired for 24 hours at 550 C. and the second wheel was fired for 24 hours at 650 C. No significant difference could be found between the two wheels in a test grinding cemented tungsten carbide.

EXAMPLE 2 Abrasive section This wheel was a cup wheel having an annulus of the dimensions described in connection with Example 1. The base section was first pressed under tons total pressure. The mold was then filled with the material of the abrasive section and the two sections were then pressed together under 150 tons total pressure. This wheel was fired at 400 F. for 3 hours then at 800 F. for 24 hours then 1000 F. for 24 hours in an atmosphere of air.

EXAMPLE 3 Abrasive section Volume Material Per Cent Diamond 120 Grit 25 Iron Powder 55 Graphite Powder The base section to go with the above abrasive section can be the same but substituting quartz for diamond. The base section should be pressed at 1 ton per square inch and the combination at 14 tons per square inch. Heating can be done in air or steam the temperature rising slowly to 1000 F. and being held thus for 24 hours.

EXAMPLE 4 Abrasive section Volume Mabem] Per Cent Diamond 120 Grit- Iron Powder ,Copper Powder. 10 Dextrine 20 The base section can be the same as the foregoing substituting quartz powder for the diamond powder using the same volume percentages. Pressure, temperature, time and atmosphere may be the same as in Example 3.

This mixture was made into a mounted point. The mounted point was a steel spindle with a steel cone at one end having molded thereon a conical abrasive shell of 1%" altitude, one half inch base diameter and a thickness of one sixteenth inch. There was no molded base section. The abrasive section was hot pressed under 10 tons total pressure for 25 minutes using a steam platen which heated the mixture to 150 C. After removing from the mold the steel spindle with the abrasive section was fired in air for 24 hours at 800 F. and then for 24 hours in air at 1000 F.

The base section was the same, substituting quartz for diamond.

This mixture was made into a "straight" disc shaped wheel 4" in diameter A" thick having a central hole, the abrasive section being A," on the radius. Two wheels were made with the above mixture, the first being hot pressed at 150 C. under a total pressure of 150 tons, then fired in air at 800 F. for 24 hours and then 1000 F.

for 24 hours. The second wheel was cold pressed under a total pressune of tons, then fired in air at 800 F. for 24 hours and then at 1000 F. for 24 hours. For this type of'wheel we prefer the hot pressing method as the chances of cracks in the green product are greatly reduced thereby.

EXAMPLE 7 Abrasive section Volume Weight- Mateflal Per Cent grams Diamond 180 Grit"... 25. 0 16.83 Iron Powder 55. 0 90. 95 Dextrine 18.5 4.08

Potentially reactive phenol-formaldehyde powder l. 5 405 Base section Volume Weight- Mateflal Per Cent grams Iron Powder 40. o 1, 909. 0 Aluminum 40.0 655.0 Dextrino l8. 5 116. 6 Potentially res v phen formaldehyde powder 1.5 11.86

The above formula was embodied in a straight disc shaped wheel 8" in diameter by thick with a central hole and the abrasive section depth (on the radius) was 1%". The base section was cold pressed at 15 tons total pressure and the total wheel was hot pressed for 35 minutes at 150 C. under a pressure of 210 tons total pressure. After removal from the mold the wheel was fired at 800 F. for 24 hours and 1000 F. for 24 hours in atmospheres of air.

EXAMPLE 8 Abrasive section Volume Weight- The above formula was embodied in a "straight disc shaped wheel in diameter, thick with an 8" central hole, the abrasive section being 1%" on the radius. The base section was pressed cold under 30 tons total pressure and the total wheel was pressed cold under 480 tons total pressure. The wheel was then fired in air for 16 hours at 500 F. and then for 24 hours at 800 F. and then finally for 24 hours at 1000 F.

The wheels of the foregoing examples are hard in grade hardness. With the exception of the one containing copper, on the Rockwell B scale all of them give readings between 90 and 115.

The following table gives the hardness of the abrasive sections of the examples where tested:

Table 1 Rockwell Hardness on B Scale Example The hardness on the Rockwell B scale of a prior art copper-tin bonded diamond wheel is about '70 to 80. The hardness on the Rockwell B scale of another prior art metal bondeddiamond wheel whose bond consists of copper, iron, tin and nickel, is about 85 to 95. The present bond is thus somewhat harder than typical metal bonds, but is not so hard as a tungsten carbide or a boron carbide bond. However the bond of the present invention wears by powdering rather than by smearing as do metal bonds. The wheel is thus free cutting and seldom needsdressing. Furthermore the bond of the invention holds the diamonds better than typical metal bonds. Wheels made according to the invention have shown a, higher rate of cut and a lower wheel wear than the best known ceramic bonded diamond wheels, grinding cemented tungsten carbide.

The accompanying drawing is a photomicrograph of a polished cross section of the abrasive section of Example 2, magnified 500 diameters. Referring to this drawing, the diamonds l are large indistinct black areas, out of focus, because polishing caused them to stand out in relief.

These diamonds l are bonded by a combination of iron oxide particles 2 (the gray areas) and also by alpha (elemental) iron particles 3 (the white areas). Pores 4 appear as small black areas in the bond 2 and 3.

5 This abrasive section of Example 2 was further the subject of an X-ray examination. An X-ray powder photograph was made with partially filtered chromium radiation. Analysis of the pattern indicated substantial amounts of F8304 (ma 1 netite) and of alpha iron (elemental iron) and diamond.

A total of specimens of bond (no abrasive) were made up in accordance with this invention in order to discover what differences resulted from 15 varying the amount of dextrine, the volume percentage of pores and of iron and firing under different conditions. Table 2 gives the volume percentage of the material and of the pores of five kinds of specimens labeled A, B, C, D and E.

Table 2 Volume Percentage Specimen Fe Dextrine Pores There were four specimens A. They were designated A-l, A-2,. A-3, and A-4. Similarly there were four specimens B and four specimens each of C, D and E. In each case the numeral indicated the firing conditions. Number 1 firing conditions involved an atmosphere of air but no attempt to introduce new air. Heating was four hours at 205 C., then 16 hours at 430 C., then 22 hours at 540 C. Number 2 firing conditions involved an atmosphere of air with slow circulati on thereof. Heating was for four hours at 205 C., for 16 hours at 430 C. and for 22 hours at 590 C. Number 3 firing conditions involved the use of a steam atmosphere with slow circulation. Heating was for four hours at 205 C., then for 22 hours at 540 C. Number 4 firing conditions involved an atmosphere of air and slow circulation thereof as in the case of Number 2 firing con- 0 ditions. However the heating in Number 4 firing conditions was a gradual rise for four hours to 540 C., then maintaining this temperature of 540 C. for 22 hours.

The various specimens were examined. Standard metallographic procedures were used for mountin and polishing. In some cases, because of the friability of the structure, no polishing could be done. Certain conclusions were drawn as the result of the examination of these speciao mens. Other conditions being equal, more of the iron is converted to iron oxide in a steam atmosphere than in an atmosphere of air (unless there is 100% conversion in air). In an atmosphereof steam, the oxidization of the iron results from disassociation of the steam freeing hydrogen.

It was found that as the amount of dextrine was raised from zero to forty volume per cent more and more of the original powdered iron was converted to oxide for a given thermal treatment. With 40% dextrine filler substantially all of the iron was converted to iron oxide when the atmosphere was air. With only 20 volume percent dextrine substantially all of the iron was converted to iron oxide when the heating was done insteam.

The more iron oxide the final bond contains, the harder it will be unless the increase in iron oxide contained is offset by the increase in porosity. Other things being equal, increasing the dextrine promotes the conversion to oxide but also increases the porosity.

With the exception of the 40 volume per cent dextrine specimens, that is specimens E, all of the specimens heated in air (firing conditions 1, 2 and 4) had:

(a) An outside shell of completely converted iron oxide,

(b) A zone of iron oxide and iron in which the iron oxide decreased in amount going toward the center, and

(c) A zone of completely unconverted iron.

Zone (b) grew at the expense of zone (c) as the amount of dextrine was increased. The specimens fired in steam (firing conditions number 3) showed far less tendency towards sharp zoning although the volume concentration of iron oxide did vary inversely with the depth of examination.

Table 3 gives the findings with respect to parand moldabllity which is why it was used in the base section in Example 2.

The graphite was provided in Example 3 to make a porous but soft acting wheel. The graphite does not burn out under the conditions involved but remains in the final product. From Table 1 it will be seen that the attempt was successful. For certain purposes a still softer cutting wheel was desired so copper powder was included. Under the conditions involved no great amount of the copper was converted to oxide. It will be seen that only 10% of copper materially lowered the Rockwell hardness, see Table 1. In Example 6 the furfural was used to make the mix more moldable and of course the furfural as well as the dextrine burned out producing pores. In Example 7 the phenol formaldehyde powder was used to increase plasticity and it also burned out with the dextrine. Furfural is a liquid and promotes mixing. The phenol formaldehyde powder does not promote mixing but makes the mixture plastic. For different sizes and types of articles furfural will be ticular specimens for comparison purposes. :5 preferred in some cases and the phenol formalde- Table 3 Approximate Volume Concentration of Phases Present Specimen 100% Iron Oxide Iron 0xide-Iron Uncoverted Mixture 1 Iron Thin ShelL. One Quarter.v Three-Quarters.

d ..do. Do.

One-Third Two-Thirds. Two-Films.-- One-Filth.

All one. Very thin shell Do. Irregular shell Do. Nearly all Do.

Disintegrated, no metallographic specimen possible.

The specimens of Table 2 and Table 3 were each of them molded discs three inches in diameter and one quarter inch thick. The abrasive sections described in the various examples hereof were all thinner than one quarter inch. Hence it is possible, by selecting combinations of atmos- -phere, temperature and time to convert enough iron to iron oxide in the abrasive section so that the major portion of the bond (including fillers such as copper or aluminum, if any) is iron oxide. In certain cases less than half of the iron in the base section will be converted to iron oxide but no detriment results therefrom. The

base section may be described as refractory material bonded with a bond selected from the group consisting of iron and its oxides and mixtures thereof.

Some comments on the eight examples may,

2 the aluminum is a filler and is not converted to the oxide except that each particle has a film of oxide which is true even before heating.

The dextrine not only produces porosity and accelerates the reaction but also makes the mixture more plastic which assists the molding'operation- The aluminum also increases plasticity hyde powder in other cases. A mixture containing as little as 2.5% furfural is tacky.

A wheel made according to Example No. 2 was given a grinding test in comparison with three other wheels of proven merit grinding cemented tungsten carbide. All wheels were the same size best quality. Wheel G was a standard coppertin bonded diamond wheel with the hard friable bond of U. S. Patent Re. No. 21,165. Wheel H was a soft copper-tin bonded wheel according to U. S. Patent No. 2,137,329. The following results were obtained.

The abrasive section of Example 1 was subjected to X-ray examination. The results confirm the findings made by X-ray examination of the abrasive section of Example 2, as above given. However in detail the results were as follows: for the wheel fired in a steam atmosphere for 24 hours at 650 0., the X-ray examination of the abrasive section of Example 1 showed the major portion of the bond to be magnetic iron oxide, F6304, with the balance substantially all alpha iron (elemental iron). Of course there was a certain porosity in the material. In the case of the abrasive .section of Example 1 which was fired for 24 hours in a steam atmosphere at 550 0., the major portion of the bond (apart from the pores) consisted of Fea04, with the balance mostly alpha iron, but with a minor proportion of ferrous oxide (FeO). It is to be assumed that under certain conditions some ferric oxide (F8203) would be developed.

In all of the examples where there was a base section, the mixture for the base section was pressed in the mold, then the top plate of the mold was removed, the mixture for the abrasive section was charged into the mold, the top plate was reinserted and the combination was pressed under higher pressure than used for the base section. In Example 5 the mixture for the abrasive section was pressed onto the cone of the steel spindle.

When dextrine was present in the mixture it burned out in the firing. Any other organic material which will leave little or no ash could be substituted for the dextrine. Besides providing a desired porosity in the article, the dextrine makes the mix plastic, it aids in molding, but as seen from Example 1 it may be omitted in certain cases.

Likewise copper helps in making the mixture plastic and isan aid to molding, but it does not burn out although it may be oxidized to a greater or less extent. In the case of Examples 2, '7 and 8 aluminum was included in the base section to make it more machinable. Other non-ferrous metals could be used in place of aluminum or copper, for example nickel or silver. As has been seen, the addition of a soft metal tends to make the final article less hard (Example 4 and Table 1) and in the case of the base more machinable. Graphite likewise makes the mixture more moldable and the final article softer.

Those skilled in the art will readily understand that the firing temperature is somewhat a function of the elapsed time of firing and vice versa. However in general it may be said that we prefer temperatures between 450" C. and 750 C. and in this range the best practical temperatures seem to be between about 500 C. and 700 C.

The iron powder used in the various examples was pure iron powder known in the art as carbonyl iron powder, that is to say derived from iron carbonyl. However the use of other iron is not precluded in certain cases, for example cast iron and/or steel powders could be used. The hard filler in the base section was quartz in many of the examples, being the material referred to commercially as flint, but other fillers could be used such as aluminum oxide or silicon carbide. We may use, as a filler in the base section, any non-metallic refractory material, and by refractory we mean such as will not fuse at the firing temperature involved. But as in Examples 2, 7 and 8 the material in the base section, besides iron, may be a metal.

It will thus be seen that there has been provided by this invention a method and an article in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made 10 in the embodiment above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

We claim:

1. The method of making a diamond abrasive article which comprises mixing together a quantity of diamond powder and a quantity of iron powder, molding the mixture of powders to form a desired shape, pressing the mixture, then heating the pressed shape in an oxidizing atmosphere to convert so much of the iron to iron oxide as will give at least 50% by volume of iron oxide in the article exclusive of the diamond powder and pores.

2. Method according to claim 1 in which the oxidizing atmosphere is air.

3. Method according to claim 1 in which the oxidizing atmosphere is steam and the heating is carried to at least 540 C.

4. The method of making a diamond abrasive article which camprises mixing together a quantity of diamond powder and a quantity of iron powder and a quantity of organic material, molding the mixture of powders to form a desired shape, pressing the mixture, then heating the pressed shape in an oxidizing atmosphere to convert so much of the iron to iron oxide as will give at least 50% by volume of iron oxide in the article exclusive of the diamond powder and pores and to burn out the organic material.

-5. Method according to claim 4 in which the oxidizing atmosphere is air.

6. Method according to claim 4 in which the oxidizing atmosphere is steam and the heating is carried to at least 540 C.

'7. The method of making a, diamond abrasive article which comprises mixing together a quantity of diamond powder and a quantity of iron powder and a quantity of graphite, molding the mixture of powders to form a desired shape, pressing the mixture, then heating the pressed shape in an oxidizing atmosphere to convert as much of the iron to iron oxide as will give at least 50% by volume of iron oxide in the article exclusive of the diamond powder and pores.

8. The method of making a diamond abrasive article which comprises mixing together a quantity of diamond powder and a quantity of iron powder and a quantity of non-ferrous metal powder, molding the mixture of powders to form a desired shape, pressing the mixture, then heating the pressed shape in an oxidizing atmosphere to convert so much of the iron to iron oxide as will give at least 50% by volume of iron oxide in the article exclusive of the diamond powder and pores.

9. The method of making a diamond abrasive article which comprises first charging into a mold a mixture of powders of non-metallic refractory material and of iron, then pressing the mixture, then charging into the mold a mixture of powders of diamond and iron, then pressing the second-named mixture onto the first-named mixture and with a greater pressure than in the first-mentioned step of pressing, then heating the combination article thus produced in an oxidizing atmosphere to convert so much of the iron that was mixed with the diamond powder to iron oxide as will give at least 50% by volume of iron oxide in the part of the combination containing diamond powder exclusive of said diamond powder and pores.

oxidizing atmosphere is steam and the 10. Method according to claim 9 in which the oxidizing atmosphere is air.

11. Method according toclaim 9 in which the heating is carried to at least 540 C. Y

12. An abrasive article comprising diamond abrasive bonded with a sintered bond comprising iron oxide, the major portion of the bond by volume consisting of iron oxide and the bond including a substantial proportion of elemental iron (alpha iron).

13. An abrasive article comprising diamond abrasive bonded with a sintered bond comprising iron oxide of which there is a substantial proportion of magnetic iron oxide F6304, the major portion of the bond by volume consisting of iron oxide including said F'e304 and the bond including a substantial proportion of elemental iron (alpha iron).

14. A grinding wheel comprising a molded sintered supporting base section comprising refractory material bonded with'a sintered bond selected from the group consisting of iron and its oxides and mixtures thereof, and an abrasive section comprising diamond abrasive bonded with a sintered bond comprising iron oxide and a substantial proportion of elemental iron (alpha iron), the base section and the abrasive section being molded into one piece and sintered together.

15. A grinding wheel comprising a molded sintered supporting base section comprising refracl2 tory material bonded with a. sintered bond selected from the group consisting of iron and its oxides and mixtures thereof, and an abrasive section comprising diamond'abrasive bonded with a sintered bond comprising iron oxide of which there is a substantial proportion of magnetic iron oxide I eaO4, the major portion of the bond in the abrasive section by volume consisting of iron oxide including said F8304 and the bond in the abrasive section including a substantial proportion of elemental iron (alpha iron), the base section and the abrasive section being molded into one piece and sintered together.

ROBERT H. HUN'IEB.

WALLACE H. FRIBERG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Ball A June 1a, 1939 

1. THE METHOD OF MAKING A DIAMOND ABRASIVE ARTICLE WHICH COMPRISES MIXING TOGETHER A QUANTITY OF DIAMOND POWDER AND A QUANTITY OF IRON POWDER, MOLDING THE MIXTURE OF POWDERS TO FORM A DESIRED SHAPE, PRESSING THE MIXTURE, THEN HEATING THE PRESSED SHAPE IN AN OXIDIZING ATMOSPHERE TO CONVERT SO MUCH OF THE IRON TO IRON OXIDE AS WILL GIVE AT LEAST 50% BY VOLUME OF IRON OXIDE IN THE ARTICLE EXCLUSIVE OF THE DIAMOND POWDER AND PORES. 